1. Field of the Invention
The present invention relates to a radio communication apparatus and, for example, to a mobile station used for a mobile radio communication system having employed the W-CDMA (UMTS) communication system.
2. Description of the Related Art
At present, efforts are continuously made by the 3GPP (Third Generation Partnership Project) for standardization of the W-CDMA (UMTS) system, which is one of the third generation mobile communication systems. The HSDPA (High Speed Downlink Packet Access), which can provide a maximum transmission rate of about 14 Mbps in the downlink, has been specified as a feature of the standardization.
The HSDPA employs adaptive modulation and coding (AMC) and is characterized, for example, by adaptive switching of the QPSK modulation scheme and the 16 QAM scheme in accordance with the radio environment between the base station and mobile stations.
Moreover, the HSDPA employs an H-ARQ (Hybrid Automatic Repeat reQuest). In the H-ARQ, when a mobile station detects an error in the data received from a base station, a retransmission request is transmitted to the base station. The base station, upon receiving this re-transmission request, executes the re-transmission of data. Therefore, a mobile station executes an error correction decoding using both the data already received and the re-transmitted received data. In the H-ARQ as described above, even if an error exists, the number of times of re-transmission is minimized by effective use of the received data.
Principal radio channels used for the HSDPA include HS-SCCH (High Speed-Shared Control Channel), HS-PDSCH (High Speed-Physical Downlink Shared Channel), and HS-DPCCH (High Speed-Dedicated Physical Control Channel).
HS-SCCH and HS-PDSCH are shared channels of the downlink (namely, transmitting information from the base station to the mobile station), while HS-SCCH is a control channel for transmitting various parameters for data to be transmitted by the HS-PDSCH. Various parameters include modulation type (Modulation Scheme) information indicating a modulation type to be used, the number of spreading codes to be assigned (number of codes), and pattern of the rate matching process to be implemented before transmission.
Meanwhile, HS-DPCCH is a dedicated control channel of the uplink (namely, from the mobile station to the base station). For example, this channel is used by a mobile station to transmit to the base station the result of acknowledgment or non-acknowledgment of reception (as the ACK signal or the NACK signal) of the data received from the base station via the HS-PDSCH channel. If a mobile station fails to receive the data (for example, if a CRC error is detected in the received data), the base station executes the re-transmission control because the NACK signal is transmitted from the mobile station as the re-transmission request. If neither the ACK signal nor the NACK signal can be received (in the case of DTX), the base station also executes the re-transmission control. Accordingly, when the DTX condition occurs, if the mobile station does not transmit the ACK signal or the NACK signal, this condition may also be considered a re-transmission request.
Moreover, HS-DPCCH is also used to transmit reception quality information (for example, SIR) of the received signal from the base station measured by the mobile station to the base station as the CQI (Channel Quality Indicator) information. The base station switches a type of transmission of downlink with the CQI information received. Namely, when the CQI information indicates that the downlink has a favorable radio environment, the type of transmission is switched to the modulation scheme enabling high-speed transmission of data. When the CQI information indicates, on the contrary, that the radio environment of the downlink is unfavorable, the type of transmission is switched to the modulation scheme for the lower transmission rate of data (namely, adaptive modulation is conducted).
[Channel Format]
Next, a channel format in the HSDPA channel will be described.
FIG. 1 illustrates a channel format in the HSDPA channel. Since the W-CDMA employs a code division multiplexing, each channel is demultiplexed with the code.
The following channels are not shown, but will be described briefly here for the sake of clarity. The CPICH (Common Pilot Channel) and P-CCPCH (Primary Common Control Physical Channel) are respectively common channels of the downlink. The CPICH is the channel used for channel estimation, cell search in the mobile station, and as the timing standard of the other physical channel of the downlink in the same cell. The P-CCPCH exists within each cell and is used for transmission of the broadcast information.
The following describes the timing relationship of channels with reference to FIG. 1. As illustrated in the figure, each channel forms one frame (10 ms) with 15 slots (each slot corresponds to the length of 250 chips). As described above, since the CPICH channel is used as the standard of the other channels, the leading parts of the frames of the P-CCPCH and HS-SCCH channels are well matched with the leading parts of the frames of the CPICH. Here, the leading part of the frame of the HS-PDSCH is delayed by two slots from the HS-SCCH channel, or the like, in order to make it possible for the mobile station to demodulate the HS-PDSCH with the demodulation scheme corresponding to a modulation type indicated by the modulation type information received via the HS-SCCH. Moreover, the HS-SCCH and HS-PDSCH form one sub-frame with three slots.
The HS-DPCCH is the uplink channel. The first slot thereof is used to transmit the ACK/NACK signal, which is the response signal for verifying reception to the base station from the mobile station after the passage of approximately 7.5 slots from the reception of the HS-PDSCH channel. The second and third slots threreof are used for periodic feedback transmission of the CQI information for the adaptive modulation control to the base station. Here, the CQI information to be transmitted is calculated on the basis of the reception environment (for example, the SIR measurement result of the CPICH) which has been measured within the period defined by the timing between the fourth slots through the first slots of the transmission preceding the CQI information.
FIG. 2 illustrates a CQI table for using received SIR (Signal to Interference Ratio) of the CPICH. As illustrated in FIG. 2, the table defines the number of TBS (Transport Block Size) bits, the number of codes, modulation type, and correspondence between CPICH and SIR respectively for the CQI information 1 to 30.
Here, the number of TBS bits indicates the number of bits transmitted within one sub-frame, the number of codes indicates the number of spreading codes used for transmission of the HS-PDSH channel, and the modulation type indicates either the QPSK and QAM methods.
As shown in the figure, the better (higher) the SIR of the CPICH channel is, the larger the value of the CQI. As the CQI value increases, the number of corresponding TBS bits and the number of spreading codes increase as well, and the modulation scheme is switched to the QAM modulation system. Accordingly, it can be understood that when the SIR is better, evidencing a more optimal radio environment, the faster the transmission rate. However, consumption of the available radio resources (spreading code or the like) increases correspondingly.
The table illustrated in the figure may be stored in the memory of the mobile station. As explained previously, the mobile station measures the received SIR of the CPICH during the receiving environment measuring period and transmits the identified CQI corresponding to the SIR measured with reference to the stored table to the base station.
The base station executes the adaptive modulation control described previously in accordance with the received CQI information to attain the transmission control analyzing the receiving environment of the CPICH in the mobile station. The channel format of HSDPA has been briefly described above. The items for HSDPA are disclosed, for example, in the 3G TS 25. 212 (3rd Generation Partnership Project: Technical Specification Group Radio Access Network; Multiplexing and channel coding (FDD) V6. 2.0 (June 2004)).
According to the prior art described above, transmission control may be efficiently realized by analyzing the radio environment between the mobile station and base station through the adaptive modulation control and re-transmission control.
However, analysis is not performed for the case of a mobile station transmitting (transferring) the data received from the base station to an external apparatus (the other apparatus connected by wire or radio to the mobile station).
For example, if the transmission (transfer) rate for the external apparatus is too slow, resulting in the data being held, the adaptive modulation control and re-transmission control are carried out without any particular analysis of the environment.
In some cases, the amount of held data exceeds the threshold value (limit value) and the data is overwritten. In this case, the re-transmission control may be conducted by an upper layer (for example, an application layer), but the re-transmission control becomes slower for the upper layer and thereby maximally efficient transmission (transfer) of data to the external apparatus cannot be realized, resulting in a drop in throughput.